Variabilidad espacio-temporal de la porosidad de la arena tras una regeneración de playa

Abstract

Coastal erosion is a natural phenomenon that is a growing problem. Over the last few decades, there has been a gradual change from hard to soft coastal defence techniques. In this sense, periodic artificial nourishment is widely regarded as an acceptable method of beach and dune protection and restoration. According to Dean (2003), after beach nourishment activities are performed, sand volumetric evolution, particularly the proportion of sand volume that remains within the region where the sand was placed, is of interest. This region can be described by features such as the pore size, density, porosity, permeability and water content. The bulk density is an indicator of soil compaction, which is related to its porosity and permeability. Beach nourishment features are based on several clues: native and borrow sand quality, dredging and dumping areas, method of transport, operating budget, sand volume needed and coastal groundwater dynamics. An important consideration in sediment motion is the relative volume occupied by sediment within a sediment bed. This is quantified using porosity or grain concentration. A standard assumption in coastal engineering is that the porosity of natural beach sand (non-cohesive) is 40%. However, is this assumption correct for all beach sand? The purpose is to demonstrate that much of the volume loss after beach nourishment procedure can be due to porosity decrease. Our primary hypothesis is that any type of beach nourishment activity results in a significant increase in sand porosity, largely due to the massive and disorganised dumping of a mixture of sediment and water (in this case 20 and 80%, respectively) on the backshore and foreshore by a trailing suction hopper dredge. The porosity subsequently decreases until it reaches its native value because of waves and tides cause spatial re-accommodation of the grains. This research proposes an accurate and simplified method to assess changes in sand porosity after beach nourishment by means of in-situ density surveys through a nuclear densimeter. This novel application has been applied to different beaches in the southwest of Spain according to the tidal range, grain size and beach morphology in several terms. Complete monitoring of the density, humidity, grain size analysis and modified Proctor compaction test was performed at all beaches over the very short, short, medium and long term (hours, days, months and years respectively). The goal of the monitoring was to study the effect of different phenomena on the internal beach sand porosity. General results show that sand porosities range from 25.6% to 43.4% after beach nourishment works, with maximum relative and cumulative differences in the order of ±40% over several terms. This research can be considered a support tool in coastal engineering to find shifting sand volumes as a result of sand porosity variability after beach nourishment and later marine influence.